Earthing

Earthing means connecting the metallic part of a machine that does not carry current under normal conitions to the earth. it is done for the protection of electrical equipment and to provide a least resistive path from the equipment to the earth. Earthing is to ensure safety or protection of electrical equipment and humans by discharging electrical energy to the earth.

Electrical Machine

Electrical distribution system is a large network that is generated, transmitted and distributed to different residential, commercial and industrial areas. The majority of these systems rely upon generators, transmission towers, starters and protecting devices. These systems are the backbone of an electrical system.

Generator

A generator transforms mechanical energy into electrical energy. Mechanical force is applied on the axle of generator. This mechanical force generated by falling of water, steam or wind, is used to rotate the shaft or axle on which the armature or rotor is formed. For example, a turbine (water wheel) that uses falling water to rotate the shaft of an electric generator.

Fleming's Left Hand Rule -When we stretch our left hand hen the forefinger, middle finger and thumb are perpendicular to each other. The forefinger represents the magnetic field, the middle finger represents the direction of current and the thumb represents the direction of force. It applies on the electric motor.

Fleming’s Right Hand Rule- When we stretch our right hand then, like the left hand, the forefinger, middle finger and thumb are perpendicular to each other. Here the forefinger represents the magnetic field, the middle finger represents the Direction V that conducting wire direction of current and the thumb represents moves the direction of force. It applies on the Magnetic field B electric generator.

An electromagnet is an artificial magnet. When electrical material like a wire is wound around the magnetic material, and current passes through the wire, it produces a magnetic field along the magnetic material. This phenomenon is due to the magnetic effect of current. Field windings are used to produce magnetic field in a generator or motor. Magnetic field can be provided by magnets or by electromagnets. Mostly, electromagnets generate the magnetic field in AC generators. Electromagnets are supplied with an external current to keep the magnetic field at its desired magnetic strength.

Types of Generator

Generators are of two types —AC generator and DC generator. AC generator is also known as an alternator. It is an electrical machine that generates AC voltage. It can be further classified as a single-phase AC generator, and a three-phase AC generator.

Working of an Alternator or AC Generator - According to the Faradays's law of Electromagnetic induction, whenever a conductor moves in a magnetic field, an electromotive force is induced across the conductor. if a closed path is provided to the conductor, induced electromotive force causes current to flow in the circuit.

DC Generator- DC generator is an electrical machine that generates DC voltage. According to Faraday’s Law of Electromagnetic Induction, whenever a conductor is placed in a varying magnetic field or a conductor is moved in a magnetic field, an emf gets induced in the conductor. The magnitude of induced emf can be calculated from the emf equation of a DC generator. If the conductor is provided with a closed path, the induced current will circulate within the path. In a DC generator, field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus, an electromagnetically induced emf is generated in the armature conductors.

In an AC generator, current is collected by using a slip ring. While in a DC generator, current is collected by a split ring. Electromotive force is an electrical force that is exerted on the electrons. This force is responsible for the movement of electrons

Armature is the rotating part of a motor or generator. Conductors are wound into the armature slots. The axle on which the armature is formed is known as a shaft. An armature (coil) is the movable coil of wire that rotates through a magnetic field. An armature (coil) may consist of many coils similar to the armature in a DC generator. A brush is a stationary or fixed part of a generator used for collecting current, which is produced due to induced electromagnetic force. Brushes in an AC generator are the sliding contacts that ride against slip rings and are used to connect the armature to the external AC circuit. Split ring (associated with DC machines) is a half ring like the brake shoes of drum brakes. The purpose of a split ring is to convert AC to DC and provide DC supply to a DC generator and to convert DC to AC and provide AC supply to a DC motor.

What is a Motor ?

An electric motor is a device that converts electrical energy into mechanical energy. This conversion is usually obtained through the generation of a magnetic field by means of current flowing into one or more coils.

Types of Motors- Motors are classified based on the electric power supply they are using for their operation that is, AC or DC.

DC Motor- DC motors require DC supply for their operation. It works on the principle that “when a current carrying conductor is placed in a magnetic field, it experiences a force”. This rotating force is called torque. DC motors are classified as brushed DC motor, and brushless DC motor.

A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source.

In a brushless DC motor, the permanent magnets are fitted to the rotor, with the electromagnets on the stator.

AC Motor- AC motors require AC supply for their operation. It works on the principle that “when a current carrying conductor is placed in a magnetic field, it experiences a force”. This rotating force is called torque. AC motors are classified as synchronous motor, and asynchronous motor. In synchronous motor, the rotating speed of the rotor is the same as the rotating speed of the magnetic field. For instance, if the magnetic field is rotating at a speed of 1000 rotations per minute (RPM) and the rotor is rotating at nearly equal, 998 rotations per minute (RPM), then the motor is said to be synchronised.

In asynchronous motor, the rotating speed of the rotor is less than the rotating speed of the magnetic field. An asynchronous motor is also known as an induction motor. The word squirrel cage defines that the structure of the rotor is like the cage of a squirrel. When a three-phase supply is connected to the stator of a squirrel cage induction motor, it produces a rotating magnetic field in the air gap between the stator and rotor, This rotating magnetic field cuts the conductors of the squirrel cage rotor. According to Faraday’s Law of Electromagnetic Induction, an emf is induced in the conductor of the squirrel cage rotor. The rotor conductors are shortened by the end ring, According to Lorentz law, if a current carrying conductor is placed in a magnetic field it will experience. a force, This collective force will make the rotor to rotate. The squirrel cage induction motor comes with some issues too. It has a low starting torque, which means it starts slowly. The reason for the low starting torque is the phase difference between voltage and current as inductive action takes place. Pure inductive effect produces a phase difference between voltage and current. In squirrel cage induction motor, total inductive effect increases due to the presence of rotor bars. These conductor bars act as inductors and produce more phase difference between current and voltage. This phase difference results in a low starting torque,

Slip Ring Induction Motor- In slip ring motor, the drawback of the squirrel cage motor is eliminated by using the slip ring in series with the rotor winding. In this section, we will discuss the slip ring motor. In slip ring motor, inductor coils are connected in series with the inductive effect of the rotor winding. This will reduce the inductive effect of the rotor as well as rectify the phase difference problem of the squirrel cage motor. three slings are used in series with the rotor winding. This will increase its starting torque of the slip ring motor. Therefore, slip ring motor is used where high stating torque is required.

Starter- Since an induction motor draws more current while starting, various starting methods are used to start the motor. By using one of the starting techniques, the winding can be saved from damage from high starting current. Different types of starters are used to safely start the induction motor. The commonly used starters are direct online starter (DOL), and star-delta starter. Direct online starter (DOL) is the simplest form of motor starter. DOL is used to safely start the induction motor. DOL consists of a circuit breaker, contactor and an overload relay for protection. In DOL starter, electromagnetic contactors are used. This contactor will open, when thermal relay is overloaded due to fault conditions. Typically, the contactor will be controlled by separate start and stop buttons .

How works DOL ?

DOL starter connects the three-phase main with the motor The control circuit is connected to any two phases to energise them. When we press the start button, the current flows through the contactor coil (magnetising oil) and c ontrol circuit. The current energises the contactor coil and leads to close the contacts. The three-phase supply is then available for the three-phase motor. When we press the stop button, the current through the contact is discontinued, making supply to the motor unavailable and stopping the machine. Star delta starter helps to reduce initial voltage induction as in a three-phase induction motor high starting current is drawn by the motor and this could destroy the stator winding. To reduce initial voltage induction, the motor has to be connected in a star manner. After it achieves 1/3 of its maximum speed, it will switch to delta configuration. For this, a star-delta starter is needed. A star-delta starter will initially make the motor run in a star form, and then switch to delta.